The present invention relates to generally to signalling apparatuses, and more particularly to a signalling apparatus for use in an ATM (Asynchronous Transfer Mode) switching system.
An ISDN (Integrated Services Digital Network) is provided with two types of channels, namely, information channels (or B or H channels) for transfer of user's information, and signalling channels (or "D channels") for transfer of control signals for setting channels.
A layered data processing scheme utilizes a three layer structure. In layer 2 functions commonly accomplished by LAPD (Link Access Procedure on the D-channel) devices are accomplished. In layer 3, call control in accordance with the I.451 standard is accomplished.
In an earlier method of processing signalling channels in an ISDN switching system, signalling channels from a plurality of subscriber lines are separated from information channels. Only the signalling channels are multiplexed and guided to a signalling processor. Thereafter, the processes of layer 2 (LAPD) and layer 3 (call control) are performed. See for example, FIG. 12 of "Signalling Processing in an I Interface Switching System", IEICE (The Institute of Electronics Information and Communication Engineers) Technical Report SE 87-85, or Japanese Laid-Open Patent Application No. 131652/1987.
The International Telegraph and Telephone Consultative Committee ("CCITT") is now considering a "broadband ISDN" as the next generation ISDN. The broadband ISDN is an attempt to provide switching services at a line rate of, for example, 150 Mbit/s. CCITT Draft Recommendation I.121 cites the ATM (Asynchronous Transfer Mode) as a favorable means for implementing the above-mentioned broadband ISDN.
The ATM switching system is a system in which all data, including control signals and information, are transferred in fixed-length packets called "cells". For example, when a signalling frame 50 is to be transmitted from a subscriber's terminal, as shown in FIG. 5(A), the frame 50 is segmented into fixed length cells 51A-51N, which cells are then transmitted through the subscriber line. An ADP processing unit in the switching system will then reassemble the signal frame 50 from the cells 51A-51N received thereby, and LAPD-LSI 102 performs layer 2 functions for a signally channel (which is called "LAPD"). Signalling processor 40 processes functions of layer 3 for a signally channel (which is called "call control"). When, on the other hand, a signal frame is to be transmitted from the switching system to a subscriber's terminal, the switching system will perform such segmenting and the subscriber' terminal will perform such a reassembly procedure.
The processes of segmenting/reassembling are determined to be functions of the "adaptation layer" in accordance with the CCITT Draft Recommendation I.121. The processing unit of the adaptation layer, including the segmenting/reassembling processes, is hereinafter called "ADP processing unit."
The arrangement of the above-stated ATM switching system is constructed in a similar manner to conventional ISDN switching system, and the processing of signalling therein, are described with reference to FIG. 9. In this figure, 11 (11-1-11-j) designates optical fibers having a transmission rate of, for example, about 150 Mbit/s; 10 (10-1-10-j) designates subscriber line interfaces which perform, for example, optical/electrical signal conversion; 30 designates an ATM switch for switching cells: 12 (12-1-12-j) designates 8-bit buses.
Cell dropper/inserter circuits 16 (16-1-16-j) are inserted between the subscriber line interfaces 10 and the ATM switch 30. In receiving subsystems (from the subscriber line interfaces 10 to the ATM switch 30), only cells in the signalling channels are separated and extracted, while in transmission subsystems (from the ATM switch 30 to the subscriber line interfaces 10), signalling channel cells are inserted. Cell data from the cell dropper/inserter 16 is multiplexed by a multiplexer/demultiplexer (MUX) 17, and connected to a subscriber line signalling unit 1 via a bus 19. The subscriber line signalling unit 1 comprises: a multiplexer/demultiplexer (MUX) 18, for multiplexing and demultiplexing cell data for each subscriber's line; the above-mentioned ADP processing units 100 (100-1-100-j); LAPD-LSI 101 (101-1 101-j), for processing layer 2 signalling channels; and a bus 103.
In accordance with the above-mentioned system, a drawback exists in that one cell dropper/inserter 16 is necessitated for each subscriber's line, and MUX 17 and buses for connecting them are further required, since only cells in the signalling channels are collected. Further, the MUX 18 and ADP processing units for corresponding subscriber's lines must be provided in the subscriber line signalling unit 1.
An arrangement in which the subscriber line signalling apparatus and subscriber's terminals are interconnected by a switch is disclosed, for example, in Japanese Laid-open Patent Application No. 78637/1988. Such a system does not, however not contemplate any adaptation process and circuitry, such as a scanning unit, for subscriber lines necessary to detect signal frames.
An advantage of the present invention is the provision of a system which provides a signalling apparatus for an ATM switching system which minimizes equipment requirements, thereby providing reduced cost, smaller size, and increased reliability.